1. Field of the Invention
The present invention relates to an output buffer amplifier circuit, and more specifically to a gain controllable output buffer amplifier circuit to be incorporated in a semiconductor integrated circuit.
2. Description of Related Art
Some typical output buffer amplifier circuit of this type known to the inventor includes an operational amplifier having an inverting input connected through an input resistor to a signal input terminal, and an output buffer amplifier having a non-inverting input connected directly to an output of the operational amplifier and an output connected to an output terminal. A tapped feedback resistor is connected between the output and the inverting input of the operation amplifier, and each of a plurality of taps of the feedback resistor is connected through a corresponding transistor switch to the output of the operation amplifier. The transistor switches are controlled by a switch control circuit so that the transistor switches are selectively turned on and off.
In the above mentioned output buffer amplifier circuit, an amplification factor, namely a ratio of an output signal to an input signal, is determined by a ratio of an effective resistance of the feedback resistor to a resistance of the input resistor. Therefore, since the effective resistance of the feedback resistor can be adjusted by selectively turning on the transistor switches by action of the switch control circuit, the gain of the overall output buffer amplifier circuit can be controlled.
In the above mentioned output buffer amplifier circuit, each of the transistor switches for determining the amplification factor is connected between the output of the operational amplifier and the corresponding tap of the feedback resistor. Since the operational amplifier is ordinarily designed to have a low output impedance, a current flows from the selected tap of the feedback resistor through the turned-on transistor switch to the output impedance of the operational amplifier. Accordingly, the amplification factor is not determined by only the feedback resistor, but determined by the ratio between the input resistance and an effective feedback resistance including the feedback resistor and the on-resistance of the transistor switch. The on-resistance of the transistor switch varies dependently upon a voltage applied across the transistor switch. Since the voltage applied across the transistor switch varies dependently upon the input signal level, the amplification factor accordingly varies dependently upon the input signal level.
In this connection, it can be considered that the variation of the amplification factor can be reduced or prevented by minimizing the variation of the voltage applied across the turned-on transistor switch in comparison with the feedback resistance. For this purpose, however, it is necessary either to enlarge the feedback resistor or to scale up the size of the transistor switches. This means that the size of the circuit components becomes large, and therefore, this way cannot be adopted in certain applications of the output buffer amplifier circuit. In particular, the output buffer amplifier circuit has to be small when the circuit is implemented in an integrated circuit.